Magnetic recording media such as audio cassette tape and video cassette tape have conventionally been used or stored in environments with a mild temperature change, for example in rooms. With the recent spread of radio-cassette players, automobile audio sets, small-sized video tape recorders, etc., use of magnetic recording media in high temperature or low temperature environments such as outdoors or in automobiles has been extending. For example, it was reported that the highest maximum of the temperature in automobiles reaches 104.degree. C. at daytime in high summer according to JIS D0204 (High and low temperature test method of automobile parts).
When exposed to such high temperatures, magnetic recording media undergo curling due to shrinkage. Shrinkage may occur in one edge to a greater degree to cause arching in the lengthwise direction, or the increased winding force due to shrinkage causes deformation of the hub. Thus, if a magnetic recording medium on which music, for example, is recorded is exposed to a high temperature, the output level on reproduction greatly varies, failing to normally reproduce the recorded signals. Besides, the tape refuses to run smoothly. These problems arising from shrinkage are particularly conspicuous in magnetic recording media in which a thin support is used so as to have an increased length to be packed in an audio or video cassette shell (sometimes called cassette half) and the number of turns in winding up around the hub is so much increased, such as audio compact cassettes C-80, C-90 and C-120.
Further, magnetic recording media using a flexible support having too a smooth surface turned out to have poor running performance on repeated use.
Various proposals have hitherto been made to provide a magnetic recording medium which has satisfactory running properties on repeated use and undergoes no great deformation due to thermal shrinkage on exposure to high temperatures thereby assuring output level stability and dimensional stability. For example, JP-A-59-132421 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposed a magnetic recording medium using a support having a surface roughness (Ra) of from 0.01 to 0.1 .mu.m on the side opposite to a magnetic layer and having a thermal shrinkage of not more than 1% when allowed to stand in an atmosphere of 110.degree. C. for 4 hours. The proposal has accomplished some improvement on thermal shrinkage of magnetic tapes or supports.
However, it has been found by further studies that the surface roughness (Ra) of a support should be more strictly limited to a range of from 0,020 to 0.070 .mu.m, and preferably from 0.03 to 0.060 .mu.m to obtain consistency between stable running and high output and that commercially available audio tapes comprising a polyester film having such a limited surface roughness undergoes large variations in high-frequency output. The terminology "high-frequency output variations" as used herein means output variations in the lengthwise direction of tape when signals of 10 KHz and -20 dB are recorded and reproduced. The high-frequency output variation gives such an auditory phenomenon that the volume in the high key periodically changes. According to studies of the present inventors, there is no auditory problem as far as the amplitude of high-frequency output variations is within 1.0 dB, and preferably within 0.5 dB.